Radiation of various forms, for example x-rays, laser light, and microwaves, as well as particle beams of, for example, neutrons, electrons, and protons, have been used to treat tumors. Unfortunately, such radiations are not generally very specific for the tumor. Tumoricidal doses of radiation often results in serious damage to normal tissue, thus limiting irradiation to lower doses that are not curative.
Radiosensitizer compounds are drugs that act in combination with radiation to produce improved response, usually by making DNA more susceptible to radiation, or extending the life of free radicals produced by the radiation. Another type of radiation enhancer includes elements or compounds that interact directly with the radiation to cause more tissue damage by increasing the absorption or scattering of the radiation, causing more local energy deposition by production of secondary electrons, alpha particles, Auger electrons, ionizations, fluorescent photons, and free radicals, for example. For cancer therapy, the purpose is to selectively enhance dose to the tumor, so these drugs, elements or compounds must be preferentially accumulated in tumor tissue or the tumor tissue must respond in a preferential way, to spare normal tissue.
Some radiosensitizers are themselves anti-cancer chemotherapeutic drugs that appear to work synergistically with x-irradiation. For example, the drug etanidazole was found to enhance radiation-induced cell death in one cell type, but not in another. (Inanami O, Sugihara K, Okui T, Hayashi M, Tsujitani M, Kuwabara M., “Hypoxia and etanidazole alter radiation-induced apoptosis in HL60 cells but not in MOLT-4 cells,” Int J Radiat Biol 78:267-74, 2002). Carboplatin, cisplatin, and oxaliplatin were found to enhance radiotherapy in a leukemia mouse model (Dionet C A, Rapp M, Tchirkov A, “Comparisons of carboplatin and cisplatin as potentiators of 5-fluorouracil and radiotherapy in the mouse L1210 leukaemia model,” Anticancer Res 22:721 -725, 2002; Cividalli A, Ceciarelli F, Livdi E, Altavista P, Cruciani G, Marchetti P, Danesi D T, “Radiosensitization by oxaliplatin in a mouse adenocarcinoma: influence of treatment schedule,” Radiat Oncol Biol Phys 52:1092-1098, 2002).
Pre-irradiation exposure to some nucleic acid base derivatives, e.g., halogenated purines or pyrimidines, has been found to potentiate radiation treatment. Although the mechanisms are not completely understood, some data suggest that these bases may be incorporated into DNA or RNA leading to functional disruption, or that they may inhibit enzymes, such as thymidylate synthetase, thus disrupting DNA synthesis. Since gamma rays are believed to disable cell division by causing double-strand DNA breaks, DNA repair pathways are therefore important modulators of lethality. Inhibition of double strand break rejoining has been observed in human adenocarcinoma cells following protracted pre-irradiation treatment with 5-fluorodeoxyuridine. This effect appeared to be primarily related to the inhibition of thymidylate synthetase and the resulting perturbation of nucleotide pools in those cells (Bruso, CE, Shewach, DS, Lawrence, T S, Int. J. Radiat. Oncol. Biol. Phys. 19:1411-1417, 1990). 5-Fluorouracil is often used in combination with radiotherapy for the treatment of malignant tumors, particularly with rectal cancer (Buchholz, D J, Lepek, K J, Rich, T A, and Murray, D, Int. J. Radiat. Oncol. Biol. Phys. 32:1053-1058; 1995). 5-iododeoxyuridine and 5-bromodeoxyuridine are being used in clinical trials, but have shown only limited efficacy. 5-chloro-2′-deoxycytidine co-administered with three biomodulators of its metabolism, N-(phosphonacetyl)-L-aspartate (PALA), tetrahydrouridine, and 5-fluoro-2′deoxyctidine, combined with fractionated x-ray treatments, produced improved results in test animal tumor models showing a threefold dose enhancement and a substantial number of cures (Greer, S, Schwade, J, and Marion, H S, Int. J. Radiat. Oncol. Biol. Phys. 32:1059-1069; 1995). These chemical methods of radiation enhancement are not widely used in standard clinical radiotherapy treatments because of the unpredictability and variability of response in different tumor types, the toxicity of the agents, and some disappointing clinical trials.
Photophrin and similar compounds have been found useful in treating some particular types of tumors. These compounds absorb visible light and result in formation of toxic free radicals. Direct cytotoxicity via intracellular damage can result from a type I mechanism where highly reactive free radicals are generated that can damage intracellular membranes and mitochondria, or a type II mechanism where excitation to reactive singlet oxygen species leads to oxidation of membranous cell structures. Ischemic necrosis can also result by vascular occlusion occurring secondary to vasoconstriction, platelet activation/aggregation, and intravascular thrombosis, which may be partly mediated by local release of thromboxane A2. A disadvantage of this phyotodynamic therapy (PDT) is that it requires visible (laser) light to penetrate the tumor and is thus limited to superficial tissue, or those tissues that are optically accessible, generally superficial malignancies. Uniformity of dose delivery is also a problem due to the high absorbance of the light by tissue. These restrictions limit the use of PDT to only a few tumor types and situations.
Another form of radiation enhancement studied is the use of boron-10-containing compounds that have a high cross section for absorption of neutrons. Upon neutron capture, boron fissions into a lithium ion and alpha particle which have ranges of 5-9 microns and can locally damage DNA and kill cells. This is called Boron Neutron Capture Therapy (BNCT). For example, see: Miura M, Morris G M, Micca P L, Lombardo D T, Youngs K M, Kalef-Ezra J A, Hoch D A, Slatkin D N, Ma R, Coderre J A, “Boron Neutron Capture Therapy of a Murine Mammary Carcinoma using a Lipophilic Carboranyltetraphenylporphyrin,” Radiat Res. 155:603-610, 2001. This method requires a costly source of slow neutrons such as a low-power nuclear reactor or a high-current of energetic protons impinging on a cooled lithium target. Moreover, the boron-containing compounds available so far to treat human tumors do not have extraordinary affinities for tumors. These factors have hindered widespread development and use of BNCT. Target nuclides other than 10B such as 157Gd have also been proposed for neutron capture therapy.
The second type of radiation enhancement, called photoactivation, involves the use of an element that has a higher absorption coefficient for x-rays than soft tissue, which results in increased local energy deposition. For example, iodine was used in a number of studies to load the nucleic acid with the iodine-containing nucleic acid base iododeoxyuridine (IudR). After incorporation into cellular DNA in vitro, irradiation resulted in a radiotherapeutic advantage of a factor of about 3 (Nath, R, Bongiorni, P, and Rockwell, S, “Iododeoxyuridine radiosensitization by low- and high-energy photons for brachytherapy dose rates,” Rad. Res. 124:249-258, 1990). However, to achieve this, the cells were altered so that 22-45% of the thymine in their DNA had been substituted with IUdR. Unfortunately, this level of substitution is not practical in vivo, and it would be difficult to selectively make such changes only in tumor cells.
The dose enhancement of x-rays adjacent to high atomic number (high-Z) elements has been known for over 50 years. At least 20 years ago it was speculated that this effect, then noted in vitro, might be employed to enhance radiotherapy of cancer (Matsudaira, H., Ueno, A. M., and Furuno, I., “Iodine contrast medium sensitizes cultured mammalian cells to x-rays but not to γ rays,” Rad. Res. 84: 144-148, 1980). Radiation oncologists have also noted tissue necrosis around metal implants following therapeutic irradiation with x-rays (Castillo, M H, Button, T M, Doerr, R, Homs, M I, Pruett, C W, Pearce, J I, “Effects of radiotherapy on mandibular reconstruction plates,” Am. J. Surg. 156, 261 (1988)). Das and coworkers made careful measurements of the dose enhancement factor at low-Z/high-Z interfaces irradiated by x-rays (Das, I J, Chopra, K L, “Backscatter dose perturbation in kilovoltage photon beams at high atomic number interfaces,” Med. Phys. 22: 767-773, 1995). Experimental x-ray dose enhancement adjacent to bulk metallic gold was reported by Regulla and coworkers (Regulla, D F, Hieber, L B, and Seidenbusch, M, “Physical and biological interface dose effects in tissue due to x-ray-induced release of secondary radiation from metallic gold surfaces,” Rad. Res. 150, 92 (1998)). A solid state detector was placed next to a thin (150 μm) gold foil and a dose enhancement factor of more than 100 with a range of 10 μm was found in the range of 40 to 120 kV tube potential. Cells were then placed in close proximity (2 μm) to the gold surface. In a clonogenic assay, 80 keV x-rays caused 80% cell killing at 0.2 Gy, which was a factor of 50 over the control without gold.
U.S. Pat. No. 6,001,054 to Regulla and Eckhard discloses a method for treating a site in a human body to inhibit abnormal proliferation of tissue at the site by introducing a metal surface at the site and then directing ionizing irradiation to the metal surface to obtain locally enhanced radiation therapy. The metal surface can be solid, e.g., a metallic stent, which is placed in the blood vessels adjacent to the tissue to ablate. Unfortunately, it would be impractical to place bulk metal surfaces throughout all tumor vessels and tissues. The dose enhancement from the metal was observed within only about 100 microns of the stent. Therefore, a few stents in a tumor would not be enough to treat the whole tumor. The '054 patent also discloses that the metal surface can be composed of spaced apart particles which range in size from 5 μm to 100 μm in diameter and are incorporated with metal or metal ions. However, there is no showing of delivery of sufficient metal to the tumor by means of these particles for effective treatment. In addition, the form of radiation taught by Regulla is restricted to less than 400 keV, which could not treat tumors at depth. Although skin cancers might be treated using this photon energy range, such tumors are more readily removed surgically.
Herold et al. reported the use of 1.5-3.0 micrometer diameter gold particles (1% by weight) in a stirred suspension with living cells during irradiation with 100-240 kVp x-rays and found a dose enhancement factor from a clonogenic assay to be 1.54 (Herold, D M, Das, I J, Stobbe, C C, Iyer, R V, and Chapman, J D, “Gold microspheres: a selective technique for producing biologically effective dose enhancement,” Int. J. Rad. Biol. 76: 1357-1364, 2000). They also injected these particles (1.5-3 micron in diameter, 1% gold suspension) directly into a growing tumor at 3 sites followed by irradiation (8 Gy, 200 kVp). No tumor remission or shrinkage in the animals was reported, but extracted cells from the tumor were found to have a 0.15 plating efficiency rather than the control value of 0.25. Histological examination showed gold particles predominantly in the interstitial fluid, and “no gold particles were found in zones of tightly packed tumor cells, suggesting that it would be difficult to achieve uniform delivery of particles . . . throughout a tumour volume by direct injection.” Since the dose is only increased in about a 100 micron region around the gold, this method would leave many tumor cells without enhanced treatment. These investigators found the particles did not diffuse into the tumor but remained at the three sites of injection.
Prior to the present invention, there has been no report of effective treatment of tumors using radiation in combination with heavy metals. Nor has there been any attempt of using a nanoscale approach to selectively ablate the unwanted cells or tissue.